Many different kinds of anchor bolts or rock bolts are used to secure rock strata underground. One kind comprises a length of reinforcing bar having ribs along its length which enhance the anchoring ability of the bolt. In the case where a resin with a catalyst is used to secure the bolt in a hole in the rock matrix, the bolt is initially rotated in the hole to mix the resin and the ribs also serve the purpose of assisting the mixing of the resin. However, known ribbed formations on rock bolts do not protrude sufficiently to provide good mixing of the resin.
The end of the bolt which is intended to project from the rock face has a rolled thread thereon which receives a nut used to tension the bolt. The bar from which such a prior art rock bolt is made is manufactured in a steel mill and is supplied with the ribs formed on the whole length of the bar. In order to provide the rolled thread on the bolt, the bolt has to be machined to remove the ribs on a section thereof and to ensure that such section has a circular cross-section. Such a machining operation adds to the cost of the prior art rock bolt.
In certain mining operations, particularly coal mining, temporary anchor bolts are used to secure the mined face. In the case of coal mining these bolts may be made from glass reinforced resin rods. When the face carrying the rods is cut the rods are simply broken up in the cutting process. There are several disadvantages attaching to the use of resin rods. Firstly, they are relatively expensive. Also, in the case where the rod is embedded in a resin mix within a hole in the coal face, the rod, being smooth, provides poor anchoring and is also unsuitable for initial mixing of the resin in the hole.
It is an object of the invention to lessen the problems associated with prior art anchor bolts or rock bolts and to reduce the cost of their manufacture.